Competing interests: The BMJ has judged that there are no disqualifying financial ties to commercial companies. The authors declare the following other interests: CRS reports research grants paid to his institution from UK Research and Innovation, the UK Medical Research Council, the UK National Institute for Health Research, the New Zealand Ministry of Business, Innovation and Employment, the Health Research Council of New Zealand, the New Zealand Ministry of Health, and the UK Chief Scientist Office, all outside of the submitted work. CRS also reports a leadership or fiduciary role in other boards, societies, committees, or advocacy groups, paid or unpaid, with the New Zealand Government Chief Data Steward’s Data Ethics Advisory Group. JM reports none. Further details of The BMJ policy on financial interests is here: https://www.bmj.com/sites/default/files/attachments/resources/2016/03/16-current-bmj-education-coi-form.pdf.